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Structure of the Atom and Atomic Number
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Atoms are the basis of chemistry. They are the basis for everything in the Universe. You should
start by remembering that matter is composed of atoms.
According to John Dalton, atoms are tiny, indivisible, solid spheres (like billiard balls).
However, scientists have discovered that atoms are actually composed of even smaller particles
called subatomic particles.
Atoms are composed of sub-atomic particles called neutrons, electrons, and protons
You can also see that each piece has either a "+", "-", or a "0." They refer to the charge of the particle.
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The electron always has a "-" or negative charge.
The proton always has a "+" or positive charge.
If the charge of an entire atom is "0", that means there
are equal numbers of positive and negative pieces,
equal numbers of electrons and protons.
The third particle is the neutron. It has a neutral
charge (a charge of zero).
The protons and neutrons are always in the center of the atom.
Scientists call the center of the atom the nucleus. The electrons are always found whizzing around the
center in areas called orbitals.
Everything is built off of these charged particles:
You could start really small...
- Atoms
Particles make
- Elements
up atoms which
- Molecules
make up
- Macromolecules
elements
- Cell organelles
- Cells
Protons
- Tissues
Has a positive (+) electric
- Organs
charge.
Found in the nucleus of
- Systems
the atom
- Organisms
Weighs less than one
- Populations
billionth of a gram. Is
- Ecosystems
approximately 2000
- Planets
times more massive than
- Galaxies
an electron
- The Universe
Mass = 1
Neutrons
Has no electrical
charge (Neutral)
Found in the nucleus
of the atom
Weighs
approximately the
same as a proton
Mass= 1
Electron
Has a negative (-)
electric charge
Orbits the nucleus
of the atom
Weighs
approximately
1/2000 of a proton
Mass = 0
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The tiny core of an atom is called the NUCLEUS
The overall charge of the nucleus is POSITIVE
The nucleus contains both PROTONS and NEUTRONS
The electron cloud is where ELECTRONS travels in ORBITALS or ENERGY LEVELS or ENERGY
SHELLS
The outside ring is called the VALENCE shell
Atomic Number AND Mass Number
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An atoms Atomic number indicates the number of protons and electrons in a neutral atom.
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IT IS THE ATOMIC NUMBER THAT IS USED TO IDENTIFY THE TYPE OF ELEMENT
Definition
The number of protons
The number of electrons in a neutral atom
(uncharged atom).
When charged,
Example
Lithium has an atomic number of 3, has 3 protons
and 3 electrons
Mass Number
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An atom’s MASS NUMBER = the number of protons + the number of neutrons
Therefore, the NUMBER OF NEUTRONS = MASS # - # PROTONS
And NUMBER OF PROTONS = MASS # - # of NEUTRONS
Definition
The mass of the atom which includes;
Example
Lithium has a mass number of 7 (round )
The number of protons + neutrons
To find the # of neutrons subtract the Atomic
number from the Mass number
Neutrons= 7-3
=4
Standard Notation
The atomic number is found below and the mass
number is found above the element symbol
Practice Questions
Complete the following table:
Element
Hydrogen
Helium
Lithium
Oxygen
Mass Number
# of Protons
# of electrons
# of Neutrons
Standard
Notation
Charged Atoms (IONS):
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Sometimes atoms will gain or lose electrons to form an ion
An atoms that loses electrons BECOMES POISTIVE and is called a CATION
An atom that gains electrons BECOMES NEGATIVE and is called an ANION
We can determine the charge by either looking at the periodic table or by comparing the
difference between the number of protons and electron
CHARGE = PROTONS - ELECTRONS
Example:
What are the following charges?
a)
b)
c)
d)
e)
If Lithium loses 1 electron? 1+
If Oxygen gains 2 electrons? 2Aluminum gains three? 3+
Beryllium? 2+
Fluorine? 1-
ISOTOPES:
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These are atoms of the same element that have different number of NEUTRONS in the nuckeus
and different masses
The number of protons and atomic number remain the same (otherwise we would have a new
element)
Isotopes of the same element react in the same ways and have the same chemical properties
Example Chlorine has two common isotopes
Cl- 35 and Cl-37
Has the atomic mass 35 and
has 18 neutrons
Has the atomic mass 35 and
has 20 neutrons
Question:
1. Carbon has two isotopes C-12 and C-14. How many protons and neutrons do both of these
have?
Questions:
1.
Complete the table below using the information above.
Type of Particle
Charge
Mass
Protons
Neutrons
Electrons
2.
When an atom has a neutral charge, what does that say about the number of protons and
electrons?
3.
Out of the three particles which 2 have a SUBSTANTIAL mass?
4.
Label the areas of an atom.
5.
Where are the particles located? Then add the particles into the drawing above. Draw a ‘P’ for
proton, an ‘E’ for an electron and an ‘N’ for neutron.
6.
Fill in the following table;
Name of element
Symbo
l
Atomic
Number
Number
of
Protons
Number
of
Electrons
Mass #
Beryllium
Calcium
F
26
197
Neutrons
Standard
atomic
notation
Charge(when
electrons are
gained or lost)